In general, the measurement of the concentration of inorganic ions such as K.sup..sym., Na.sup..sym., Cl.sup..crclbar., HCO.sub.3.sup..sym. or CO.sub.3 .sup.2.sym., Ca.sup.2.sym., etc., in body fluids is important in the clinical field and wet procedures using ion selective electrodes have already been used in the art for this purpose for an extended time.
Known methods involving ion selective electrodes are of the type where ionic activity is measured by immersing a needle-like electrode in a liquid. It is troublesome to control such an electrode from the viewpoint of maintenance, washing, conditioning, life span, damage, etc., and it is necessary to use more than several hundred .mu.l of sample liquid since the electrode head must be sufficiently immersed in the sample liquid.
To eliminate such inconveniences, an electrode in a film or sheet form such that a sample liquid is spotted or dropped on a dry electrode in a film form is disclosed in Japanese Patent Application (OPI) No. 142584/77 (the term "OPI" as used herein refers to a patent application which has not yet been examined but is open to public inspection) and in U.S. Pat. No. 4,053,381. The film form ion selective electrode disclosed is a dry operative electrode formed by coating, on a metal layer, a layer of a water-insoluble salt of the same metal as in the metal layer, a dried electrolyte layer composed of a hydrophilic binder matrix containing dissolved therein a water-soluble salt having the same anion as that of the foregoing water-insoluble salt, and an ion selective layer on an insulating support film in this order. Two of these electrode films are connected together by a bridge to form a pair of electrodes. After connecting the electrodes to a potentiometer, a sample liquid and a reference liquid are dropped onto each of the electrodes, respectively, and the potential difference is measured, whereby the ionic activity of the sample liquid can be determined.
In these dry type ion selective electrodes, a specified ion can be measured by changing the kind of the ion selective membrane as the uppermost layer and hence there are many kinds of such electrodes, each to measure separate ion, e.g., K.sup..sym., Na.sup..sym., Cl.sup..crclbar., HCO.sub.3.sup..crclbar. or CO.sub.3.sup.2.crclbar., etc.
In the measurement of ionic activity, it is most important that a conductive layer of the ion selective electrode generate a potential response corresponding only to the ionic activity of a substance to be tested and not generate any other potential response.
A typical example involving an undesired potential response is the occurrence of an incorrect potential due to shorting. That is, when a sample liquid and a reference liquid are respectively dropped onto the foregoing electrode films, each drop of the sample liquid tends to spread over the surface of the ion selective layer (or a protective layer when a protective layer is formed on the ion selective layer) of each electrode film. The spread liquid flows down over the edge of the electrode film to short circuit various layers comprising the electrode, which results in the generation of an incorrect potential or zero potential to give an incorrect potentiometer value. Consequently, it is necessary in such ion selective electrode film to avoid such short circuiting by the sample liquid or reference liquid.
Japanese Patent Application (OPI) No. 142584/77 proposes a method of preventing the occurrence of short circuiting (hereafter often merely shorting) of the layers of an electrode film by forming a platform made of a plastic material or a strip made of a water-impermeable, electrically insulating adhesive for protecting the electrode edges in such a manner that only the ion selective layer of the electrode film is exposed and other portions of the electrode film are not permeated with or contacted by water or an aqueous liquid. However, such a procedure is labor intensive since each electrode must be so treated and it is difficult to completely avoid shorting since the processing requirements are exacting.
In U.S. Pat. No. 4,053,381 an ion selective electrode composed of three layers, i.e., a water-insoluble salt layer, an electrolyte layer and an ion selective layer, which is obtained by forming paired conductive layers with two pieces of conductive metals and forming the three layers thereon in common to the paired conductive metal layers, not forming these layers on each of the paired conductive metal layers, is disclosed. In a device for determining ionic activity as disclosed in this U.S. Patent, however, edges are inevitably formed and the device is constructed such that the disclosed film form electrodes in which edges of the electrode layers so exposed are fixed in a frame, i.e., a special frame for anti-shorting is used so that a dropped sample liquid as well as a reference liquid do not spread from the dropped area to an undesirable area (especially facing areas of two electrodes) to prevent both liquids from being brought into contact with each other to cause shorting or preventing both liquids itself from flowing down over the edge of the electrode film to short the layers of the electrode.
In order to obviate such shortcomings in the prior art, the present inventors found that when a conductive layer is covered with an ion selective layer, no particular anti-shorting means is required and a patent application directed to an invention based on this finding was filed (Ser. No. 397,705 filed July 13, 1982).
In addition to such anti-shorting means, it is important that a conductive layer of an ion selective electrode should not be in electrical contact with respect to a reference electrode. In order to avoid such electrical contact, particularly in the case of a differential method in which paired electrodes are employed, the distance between the electrodes is maintained as great as possible so that the possibility of electrical contact is lost and insulation is assured. In such case, however, the time period until a potential difference generates by diffusing both sample and reference liquids into a bridge, forming an interface between the liquids and thus achieving electrical conduction, is prolonged and rapid measurement is not possible. On the other hand, it is necessary that an ion selective electrode be as small as possible, particularly in case where paired electrode films are employed as a differential electrode or in the case where multiple ions are to be examined at the same time, it is desired that each ion selective electrode be as close as possible. Such, however, increases the danger of electrical contact between a conductive layer of one ion selective electrode and a conductive layer of another ion selective electrode, since ion selective electrodes that should avoid an incorrect response other than a potential response based on ionic activity of a substance to be tested. It has thus generally been thought impossible to realize such a demand in the prior art.
One exception is an embodiment described in U.S. Pat. No. 4,053,381 in which all necessary layers are formed on a support to yield a single element on the common support, after which electrical separation of a metal layer thereof is achieved by skiving a groove through the metal layer so that the electrode can be used for potential measurement as a paired electrode. In this case, the metal layer is electrically separated only by the skived groove and, accordingly, such construction is advantageous from the viewpoint of reducing the size of the ion selective electrode. However, after all of the operative layers of the ion selective electrode are formed the skiving of the groove is conducted so that the operative layers of the electrode, including the metal layer in question, possess edges exposed at the skived portion. As has been discussed above, there is then the problem that the exposed metal layer would short due to liquid drops flowing over the edges to thereby cause an incorrect potential. Therefore, it is necessary per such U.S. Patent that complicated anti-shorting means be provided as described above. In this U.S. Patent mention is made of continuous half cell electrode-construction layers (three layers: an ion selective layer, an electrolyte layer and a layer of a water-insoluble salt of the metal) other than a support and metal layers formed as continuous layers, respectively, but only the metal layers are separated to achieve electrical insulation. However, this construction is the same as other electrodes in that edges of the metal layers are exposed also in such a half cell and special anti-shorting must be provided.